1. Field of the Invention
This invention is concerned with the removal of atmospheric pollutants from gas streams. This invention is more particularly concerned with a method for the removal of NO.sub.x compounds from gas streams, especially from stack gases emitted from stationary combustion sources wherein nitrogen oxides are reduced to form nitrogen gases which are subsequently emitted to the atmosphere.
2. Description of the Prior Art
The Clean Air Act of 1970 directs the Administrator of the Environmental Protection Agency to promulgate national ambient air quality standards for those pollutants "which in his judgment have an adverse effect on public health and welfare." Furthermore, the Clean Air Act directs the Administrator of the Environmental Protection Agency to promulgate standards of performance with respect to the emission of air pollutants for those new stationary sources which he determines "may contribute significantly to air pollution which causes or contributes to the endangerment of public health or welfare." Nitrogen oxides (referred to herein as "NO.sub.x ", which is meant to include nitric oxide and nitrogen dioxide as well as other compounds of nitrogen and oxygen which are of less significance in the atmosphere) have been identified as air pollutants having an adverse effect on public health and welfare. Accordingly, ambient air quality standards for NO.sub.x and new source standards for various facilities such as steam generators, nitric acid plants and gas turbines have been promulgated.
In order to minimize NO.sub.x emissions from stationary combustion sources, burner design is being modified and operating conditions are being controlled. However, these measures cannot remove NO.sub.x to the low level required by EPA standards. NO.sub.x emission is particularly high for coal burning furnaces. It is known that NO.sub.x emission is related to the nitrogen content of the fuel: the higher the nitrogen content of the fuel, the higher the NO.sub.x content in the stack gas. However, denitrogenation of fuel is difficult, particularly for heavy oil and solid fuels. Furthermore, much of the NO.sub.x is derived from the nitrogen fixation reaction which is dependent on the design and operation of the furnace.
Current approaches to NO.sub.x removal from stack gases emitted by stationary combustion sources may be generally characterized as either dry contact reduction processes or wet absorption processes. An example of the former characterization is the absorption of nitrogen oxides by water or alkali solutions. These methods require large and expensive equipment such as absorption towers to treat a large volume of waste gases. Moreover, these methods are not economically effective when the NO.sub.x concentration in waste gases is 200 ppm or less.
Dry contact reduction processes may be either catalytic or non-catalytic and may be either selective or non-selective. Selective reduction methods are characterized by the selective reduction of nitrogen oxides and their consequent removal in the presence of oxygen. A known selective reduction method uses ammonia as a reducing agent. However, the ammonia reducing agent oxidizes to form nitrogen oxide at high temperatures and excessive ammonia discharged to the atmosphere is itself another source of air pollution. Other selective reduction methods employ catalysts to enhance the production of desired reduction products. For example, U.S. Pat. No. 4,016,241 suggests adding a reducing agent which is a C.sub.2 -C.sub.6 olefin or an aromatic hydrocarbon to stack gas and contracting the mixture at 200.degree.-450.degree. C. with a catalyst comprising iridium on a refractory carrier.
However, catalytic reduction processes generally have reduced effectiveness because of the presence of particulates, sulfurous acid gases and other poisons in stack gas to be treated which reduce catalyst life and thereby increase process costs.
Non-selective reduction methods generally comprise adding a reducing agent to the NO.sub.x containing gases, consuming all free oxygen present in the gases through combustion of a portion of the reducing agent, and reducing the NO.sub.x to nitrogen by the remaining reducing agent. The reduction usually occurs in the presence of a catalyst. Carbon monoxide, natural gas, methane, butane, propane, purge gas from ammonia plants, naptha and hydrogen have been employed as reducing agents. The scarcity and expense of these reducing agents has hindered wide-spread acceptance of this type of method for NO.sub.x control.
U.S. Pat. No. 3,864,450 discloses a process wherein NO.sub.x present in gaseous mixtures is reduced at temperatures between about 250.degree. C. and 480.degree. C. by contact with carbon-containing NaOH and/or KOH. Carbon used in the catalyst includes coke, semi-coke, and coal; coke, semi-coke and petroleum coke are suggested as the most suitable material.
U.S. Pat. No. 3,873,671 discloses a process wherein effluent gases are treated for NO.sub.x reduction by introducing the effluent gases to a burning fuel containing a hydrocarbon or hydrogen, maintaining the treatment zone at a temperature between 1200.degree. and 2000.degree. F. (650.degree.-1095.degree. C.) to reduce substantially all of the NO.sub.x and thereafter contacting the NO.sub.x -reduced mixture in a separate zone to convert remaining oxidizable combustion products to CO.sub.2 and H.sub.2 O without reformation of NO.sub.x. Coal is disclosed as a fuel at column 4, lines 16-22.
U.S. Pat. No. 4,060,589 discloses a process for reducing NO.sub.x (and/or SO.sub.x) components in stack gas by passing stack gas at 900.degree.-1600.degree. F. (480.degree.-870.degree. C.) over coal, coke or char obtained from coal gasification plants. The '589 patent does not suggest adding oxygen to the stack gas to be treated but does show that the stack gas contains from 0-25 percent oxygen (see Table I in column 2) and suggests that this oxygen content is important in maintaining the temperature of the reaction zone (see column 3, lines 1-45). The patent further suggests that treated gas may be post treated over a cobalt-molybdate catalyst by a known process to remove H.sub.2 S (see column 4, line 41 to column 5, line 33). The bed of carbonaceous material in the primary treatment step may be either fixed or of the moving bed type (column 7, line 62, et seq.). The use of coal as a carbonaceous material--particularly anthracite coal--is suggested in the first full paragraph of column 8. In one embodiment of the process, a portion of the coal normally used in the fuel combustion or conversion plant is carbonized and employed as a carbonaceous material in the stack treatment zone (column 8, lines 36-55).
U.S. Pat. No. 3,867,507 teaches a method for controlling NO.sub.x emissions by a thermally-activated, gas-phase chemical reaction wherein an NO.sub.x -containing combustion effluent or waste product stream is first contacted with a hydrocarbon in the presence of oxygen at a temperature ranging from 400.degree. to 2700.degree. C. for a sufficient time to substantially reduce the NO.sub.x to molecular nitrogen and then contacting the reduced stream with oxygen at a temperature ranging from 400.degree.-2700.degree. C. for a sufficient time to substantially oxidize all remaining contaminants. Hydrocarbon and oxygen (air) are added to the NO.sub.x -containing combustion effluent or waste product stream in an amount sufficient to maintain supply from 0.02 to 32 carbon atoms per molecule of NO.sub.x and less than 2.5 oxygen molecules per carbon atom. Contrasting this process from that disclosed in U.S. Pat. No. 3,873,671 described above, it should be noted that the '671 patent is essentially an "after burning" method in which another small flame is actually maintained downstream of the primary flame by injecting fuel and burning it in order to incinerate contaminating gases.
A primary object of the present invention is an improved non-selective, non-catalytic, dry contact reduction method for the removal of nitrogen oxides from NO.sub.x -containing gases. A related object is a method for controlling NO.sub.x emissions which method is compatible with the removal and control of other pullutants such as sulfur-containing gases and products of incomplete combustion.
A further object of this invention is a dry contact reduction method for the removal of nitrogen oxide from waste gases which uses inexpensive, readily available materials.
A still further object of this invention is a low-cost process for the removal and disposal of NO.sub.x from large volumes of diluent gases such as stack gases from coal fired boilers or power plants.